1. Field of the Invention
The present invention generally relates to a real time clock integrated circuit (RTC IC) and an electronic apparatus using the same, in particular, to a RTC IC having low power consumption, low fabricating cost, and low design complexity and an electronic apparatus using the same.
2. Description of Related Art
Nowadays, electronic apparatuses have been broadly applied to various aspects of our lives, and a counting function is usually disposed in an electronic apparatus for the convenience of usage. For example, a real time clock integrated circuit (RTC IC) is required for the “schedule recording” function of a video recorder or the time switch of an audio system. In addition, today's personal computers, cell phones, MP3s, personal digital assistants (PDAs), or cameras are usually disposed with RTC ICs. When an electronic apparatus is powered off, the RTC IC disposed therein still keeps counting so that when the electronic apparatus is powered on again, the RTC IC can provide correct time instantly.
However, when foregoing electronic apparatus is powered off, the external power supply to the RTC IC is stopped so that the RTC IC stops its counting function and accordingly cannot provide a correct time to the electronic apparatus instantly. Thus, to allow the RTC IC to keep counting after the electronic apparatus is powered off, a button cell Lithium battery (for example, with supply voltage of 3.0V and capacity of 50 mAH) is usually provided. Besides, thanks to the advancement of semiconductor technology, only a very small driving current is required for today's RTC IC to operate properly; for example, a foregoing Lithium battery can sustain the operation of a RTC IC for over two years.
FIG. 1 is a block diagram illustrating the internal circuit of a conventional RTC IC 100. Referring to FIG. 1, generally speaking, a crystal oscillator 101 is used in the RTC IC 100 for providing a precise standard clock as the counting reference of a real time counter 103. However, some extra control logic circuits have to be disposed in the RTC IC 100 in order to provide the counting required by various circuits in the electronic apparatus, and these additional control logic circuits in the RTC IC 100 may cause increases in both the power consumption and fabrication cost of the RTC IC 100.
As described above, the increase in the power consumption of the RTC IC 100 will shorten the lifespan of foregoing button cell Lithium battery. In order to prolong the counting operation duration of the RTC IC 100, the power consumption thereof has to be reduced. Presently, a most common method is to divide the frequency of the standard clock provided by the crystal oscillator 101 with a frequency divider 102 so as to produce a standard clock with appropriate frequency. According to the method described above, both the power consumption and fabrication cost of the RTC IC 100 are reduced.
It should be mentioned here that not only the crystal oscillator 101 provides a precise standard clock, but the precision of the standard clock it provides is not affected by temperature, humidity, process, operation voltage etc, thus, the crystal oscillator 101 is usually applied to electronic apparatuses which require precise timing. However, because the crystal oscillator 101 consumes more driving current on the foregoing button cell Lithium battery, the duration of the RTC IC 100 performing counting operation is shortened. Besides, the fabrication cost of the crystal oscillator 101 is higher than that of other common oscillators (for example, ring oscillator, RC oscillator, relaxation oscillator, etc).
In order to reduce the power consumption of the RTC IC, a “Power Down System and Method for Integrated Circuit” is disclosed in U.S. Pat. No. 6,873,215, wherein both a crystal oscillator and a low-power oscillator are adopted. The crystal oscillator is turned on when the RTC IC is in an operation mode (namely, an external power is supplied to the RTC IC), and the low-power oscillator is turned on when the RTC IC is in a sleep mode (namely, there is no external power supplied to the RTC IC). Since the low-power oscillator consumes less driving current on the button cell Lithium battery, the duration of the RTC IC performing counting operation is prolonged.
However, the precision of the standard clock provided by the low-power oscillator is easily affected by environmental factors (for example, temperature, humidity, process, operation voltage etc). Thus, according to the technique disclosed in U.S. Pat. No. 6,873,215, a sensor is further disposed in the RTC IC for measuring the standard clock provided by the low-power oscillator, and when the standard clock provided by the low-power oscillator produces a frequency drift, the standard clock provided by the crystal oscillator is used for compensating the standard clock provided by the low-power oscillator. However, by directly measuring the standard clock provided by the low-power oscillator with the sensor, both the design complexity and the fabrication cost of the RTC IC are increased.